Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2002-08-07
2004-05-25
Solola, Taofiq (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06740762
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a process for producing ascorbic acids. Specifically, the present invention pertains to a process for preparing ascorbic acids from 2-keto-hexonic acids in the presence of alkaline earth silicates as catalysts.
BACKGROUND OF THE INVENTION
L-Ascorbic acid (vitamin C) is produced commercially by a combination of chemical and fermentation processes starting from glucose or sorbose. These processes first produce a 2-keto-hexonic acid which subsequently may be cyclized through a lactonization reaction to an ascorbic acid. For example, a common 2-keto-hexonic acid generated in the commercial ascorbic process is 2-keto-L-gulonic acid (2-KLG) or its protected form, diacetone-2-keto-L-gulonic acid. The 2-keto-L-gulonic acid then may be converted to L-ascorbic acid by esterification with methanol, followed by cyclization using stoichiometric amounts of a base, in a procedure originally reported by Reichstein (T. Reichstein and A. Grussner,
Helv. Chim. Acta
17, (1934), p. 311-328). A stoichiometric amount of base is normally required for cyclization of the ester because an equivalent of base is consumed to generate the corresponding salt of L-ascorbic acid. The formation of this salt requires a subsequent acidification step to liberate the desired L-ascorbic acid product.
Improvements to the Reichstein process have often involved removal or simplification of many of the chemical processing steps required for the production of 2-keto-L-gulonic acid. Examples of such improvements include controlled esterification of 2-keto-L-gulonic acid and subsequent removal of unesterified starting material, as described in U.S. Pat. No. 5,128,487, and integration of esterification and lactonization steps, described in U.S. Pat. No. 5,391,770.
Esterfication may be avoided by conducting the lactonization in the presence of acid catalysts as described in U.S. Pat. No. 2,462,251; UK Patent Application No. 2,034,315 A; International Patent Application No. 99/07691; International Patent Application No. 00/46216, and Crawford et al.
Adv. Carbohydrate Chemistry
, 37 (1980), pp. 79-155. With acid catalysts, 2-keto-hexonic acids, their corresponding acetals, or ketals may be cyclized directly, with consecutive lactonization and enolization, to form ascorbic acids. For example, 2-keto-L-gulonic acid may be cyclized in the presence of acid catalysts to produce L-ascorbic acid. This process eliminates the need for the generation of the ester and subsequent steps requiring the addition of stoichiometric base for cyclization with reprotonation of the ascorbate salt. Alternatively, diacetone-2-keto-L-gulonic acid may be cyclized, with loss of acetone, to form L-ascorbic acid. Direct cyclization of diacetone-2-keto-L-gulonic acid, however, requires extensive purification for recovery of the acetone and other by-products generated. Additional modifications have been described to improve acid catalyzed processes such as the use of organic solvents and surfactants as described in U.S. Pat. No. 5,744,618; International Patent Application No. 87/00839; and Japan Patent No. 73015931. Although these modification have provided improvements to the original Reichstein process, significant handling, recycling, and purification steps are needed to obtain a high yield of ascorbic acid.
The lactonization of 2-keto-hexonic acids also has been accomplished using solid acid catalysts. UK Patent No. 1,222,322 discloses the use of an acidic resin catalyst for conversion of 2-keto-hexonic acids to ascorbic acids. A similar use of acidic zeolites is described in German Patent Application No. 3843389 A1. These processes, however, cannot be operated in water and require the use of non-reactive organic solvents to prevent hydrolysis of 2-keto-hexonic acid ester starting materials. Additional processing steps, therefore, are required to remove, dispose of, and recycle the organic solvents. In addition, the yields from acid catalyzed processes are often low because of decomposition of the ascorbic acid.
It is also known that ascorbic acid may be prepared from 2-keto-L-gulonic acid without the use of strong acid catalysts (see T. Reichstein,
Helv. Chim. Acta
17, 1934, pp. 311-328 and UK Patent No. 428,815) by thermal cyclization in water (U.S. Pat. No. 2,491,065) or by heating 2-keto-L-gulonic acid in water saturated with carbon dioxide (U.S. Pat. No. 2,265,121). Although avideing extensive purification steps, these processes provide uneconomically low yields of ascorbic acid.
It is evident that the existing process for converting 2-keto-hexonic acids, including 2-keto-L-gulonic acid, and their derivatives to ascorbic acids suffer either from low yields or require the use of organic solvents and extensive processing to obtain ascorbic acid products of acceptable purity. Thus a need exists for an efficient and economical process for the preparation of ascorbic acids from 2-keto-hexonic acids that avoids many of the problems inherent in the aforementioned processes. In addition, it would be useful to develop a method of catalytic ring closure for the conversion of 2-keto-hexonic acids and their derivatives to ascorbic acids that avoids the generation of by-product salts and eliminates the need for neutralization and handling of salt by-products. Even more advantageous would be the development of a process employing a heterogeneous catalyst that would enable simple and rapid separation of the catalyst from the product mixture and, thus, provide a means to conduct the preparation of ascorbic acids in a continuous manner.
SUMMARY OF THE INVENTION
The present invention provides a process for the preparation of ascorbic acids from 2-keto-hexonic acids or 2-keto-hexonic acid derivatives using basic, heterogeneous catalysts. Thus, one embodiment of the present invention is a process for preparing an ascorbic acid comprising contacting a 2-keto-hexonic acid, a 2-keto-hexonic acid derivative, or a mixture thereof with a catalyst comprising at least one alkaline earth silicate. Our process is suitable for a wide range of ascorbic acid precursors including 2-keto-hexonic acids, acetals, ethers, ketals, and esters. The alkaline earth silicate catalysts of the present invention are widely available, may be obtained at low cost, are operable over a wide temperature range, and can readily regenerated if necessary by high temperature or oxidative regeneration. Because the present invention employs a heterogeneous catalyst, separation of the product mixture from the catalyst is simple and rapid. Our invention may be carried out in water or in a variety of polar or moderately polar solvents or solvent mixtures and provides for simple workup and purification of ascorbic acid products. Our process generates no by-product salts and, thus, does not require the steps of neutralization or the handling of copious amounts of salt by-products.
Another embodiment of the present invention pertains to a process for the continuous preparation of L-ascorbic acid comprising the steps of (i) continuously feeding a water, methanol, or ethanol solution comprising a 2-KLG reactant selected from the group consisting of 2-KLG, the methyl ester of 2-KLG, the ethyl ester of 2-KLG, and mixtures thereof to a reactor operated under substantially plug-flow conditions and maintained at reaction conditions of temperature and pressure; (ii) contacting the feed solution from step(i) with a catalyst selected from the group consisting of a calcium silicate, a calcium-magnesium silicate, a magnesium silicate, and mixtures thereof wherein a portion the 2-KLG reactant is reacted to form L-ascorbic acid; (iii) continuously removing a solution comprising L-ascorbic acid and unreacted 2-KLG reactant from the reactor; (iv) separing and recovering the unreacted 2-KLG reactant from the L-ascorbic acid to produce an enriched L-ascorbic acid and a solution comprising the 2-KLG reactant; and (v) recycling the recovered 2-KLG reactant solution of step(iv) to step(i). The present invention provides for continuous production of L-ascorbic acid which
Crain Allen Lynn
Zoeller Joseph Robert
Eastman Chemical Company
Graves, Jr. Bernard J.
Middlemas Eric D.
Solola Taofiq
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